Three-Dimensional Configurations Providing Electromagnetic Interference Shielding For Electronics Enclosures

ABSTRACT

The present invention provides a configuration of an electronics enclosure, including a computer chassis in which three-dimensional shapes that may be in the form of a partial or quarter-sphere or cube or other periodic “patterns” may be stamped, molded, cut, or extruded into a lid and a five-sided “box” to provide improved EMI shielding, such that the need for gaskets is reduced or eliminated.

REFERENCE TO PRIORITY DOCUMENTS

The present application claims priority to U.S. patent application Ser.No. 11/270,827, filed Nov. 10, 2005 and entitled THREE-DIMENSIONALCONFIGURATIONS PROVIDING ELECTROMAGNETIC INTERFERENCE SHIELDING FORELECTRONICS ENCLOSURES which is a continuation-in-part and claimspriority under 35 USC §120 to U.S. application Ser. No. 11/080,385:entitled “Reduced cost and gasketting “one-hit” and other manufacturingEMI-shielding solutions for computer enclosures” filed Mar. 15, 2005,invented by Paul Douglas Cochrane. This application also claims priorityunder 35 USC §119(e) to U.S. Provisional Application Ser. No.60/642,694, filed Jan. 10, 2005, entitled Gasketless “one-hit”electromagnetic interference (EMC) shielding solutions for themanufacture of computers chassis and other electronic componentcontainers” by Paul Douglas Cochrane. All of the above-reference patentapplications are incorporated by reference, for all purposes.

BACKGROUND

The following background section is, in part, reprinted from “DesignTechniques for EMC—Part 4 Shielding” by Eur Ing Keith Armstrong, CherryClough Consultants, Associate of EMC-UK.

A complete volumetric shield is often known as a “Faraday Cage”,although this can give the impression that a cage full of holes (like MrFaraday's original) is acceptable, which it generally is not. There is acost hierarchy to shielding which makes it commercially important toconsider shielding early in the design process. Shields may be fittedaround the following: individual ICs—example cost 25P; segregated areasof PCB circuitry—example cost £1; whole PCBs—example cost £10;sub-assemblies and modules—example cost £15; complete products—examplecost £100; assemblies (e.g. industrial control and instrumentationcubicles)—example cost £1,000; rooms—example cost £10,0000; andbuildings—example cost £100,000.

Shielding always adds cost and weight, so it is always best to use theother techniques described in this series to improve EMC and reduce theneed for shielding. Even when it is hoped to avoid shielding altogether,it is best to allow for Murphy's Law and design from the very conceptionso that shielding can be added later if necessary. A degree of shieldingcan also be achieved by keeping all conductors and components very close

sheet. Ground-planed PCBs populated entirely by

mounted devices are therefore are recommended for their EMC advantages.

A useful degree of shielding can be achieved in electronic assembliesfirstly, by keeping their internal electronic units and cables veryclose to an earthed metal surface at all times, and secondly, by bondingtheir earths directly to the metal surface instead of (or as well as)using a safety star earthing system based on green/yellow wires. Thistechnique usually uses zinc-plated mounting plates or chassis, and canhelp avoid the need for high values of enclosure SE.

Many textbooks have been written on the subject of how shields work, andit is not intended to repeat them here. However, a few broad conceptswill help. A shield puts an impedance discontinuity in the path of apropagating radiated electromagnetic wave, reflecting it and/orabsorbing it. This is conceptually very similar to the way in whichfilters work—they put an impedance discontinuity in the path of anunwanted conducted signal. The greater the impedance ratio, the greaterthe SE.

At thicknesses of 0.5 mm or over, most normal fabrication metals providegood SE above 1 MHz and excellent SE above 100 MHz. Problems with metalshields are mostly caused by thin materials, frequencies below 1 MHz,and apertures.

It is generally best to allow a large distance between the circuits thatare shielded and the walls of their shield. The emitted fields outsideof the shield, and the fields that the devices are subjected to, willgenerally be more “diluted” the larger the shielded volume.

When enclosures have parallel walls opposite each other, standing wavescan build up at resonant frequencies and these can cause SE problems.Irregular shaped enclosures or ones with curved or non-parallel wallswill help prevent resonances. When opposing shield walls are parallel,it is desirable to prevent resonances from occurring at the samefrequencies due to width, height, or length. So, in order to avoid cubicenclosures, rectangular cross-sections can be used instead of squareones, and it is preferable to avoid dimensions that are simple multiplesof each other. For example, if the length is 1.5 times the width, thesecond resonance of the width should coincide with the third resonanceof the length. It is preferable to use irrationally ratio'd dimensions,such as those provided by the Fibonacci series.

come in two flavours: electric (E) and magnetic (M). Electromagneticfields consist of E and M fields in a given ratio (giving a waveimpedance E/M of 377 in air). Electric fields are easily stopped by thinmetal foils since the mechanism for electric field shielding is one ofcharge re-distribution at a conductive boundary; therefore, almostanything with a high conductivity (low resistance) will present suitablylow impedance. At high frequencies, considerable displacement currentscan result from the rapid rate of charge

even thin aluminium can manage this well

fields are much more difficult to stop. They need to generate eddycurrents inside the shield material to create magnetic fields thatoppose the impinging field. Thin aluminium is not going to be verysuitable for this purpose, and the depth of current penetration requiredfor a given SE depends on the frequency of the field. The SE alsodepends on the characteristics of the metal used for the shield which isknown as the “skin effect”.

The skin depth of the shield material known as the “skin effect” makesthe currents caused by the impinging magnetic field to be reduced byapproximately 9 dB. Hence a material which was as thick as 3 skin depthswould have an approximately 27 dB lower current on its opposite side andhave an SE of approximately 27 dB for that M field.

The skin effect is especially important at low frequencies where thefields experienced are more likely to be predominantly magnetic withlower wave impedance than 377Ω. The formula for skin depth is given inmost textbooks; however, the formula requires knowledge of the shieldingmaterial's conductivity and relative permeability.

Copper and aluminium have over 5 times the conductivity of steel, so arevery good at stopping electric fields, but have a relative permeabilityof 1 (the same as air). Typical mild steel has a relative permeabilityof around 300 at low frequencies, falling to 1 as frequencies increaseabove 100 kHz. The higher permeability of mild steel gives it a reducedskin depth, making the reasonable thicknesses better than aluminium forshielding low frequencies. Different grades of steels (especiallystainless) have different conductivities and permeabilities, and theirskin depths will vary considerably as a result. A good material for ashield will have high conductivity and high permeability, and sufficientthickness to achieve the required number of skin-depths at the lowestfrequency of concern. 1 mm thick mild steel plated with pure zinc (forinstance 10 microns or more) is suitable for many applications.

It is easy to achieve SE results of 100 dB or more at frequencies above30 MHz with ordinary constructional metalwork. However, this assumes aperfectly enclosing shield volume with no joints or gaps, which makesassembly of the product rather difficult unless you are prepared toseam-weld it completely and also have no external cables, antenna, orsensors (rather an unusual product). In practice, whether shielding isbeing done to reduce emissions or to improve immunity, most shieldperformance are limited by the apertures within it.

Considering apertures as holes in an otherwise perfect shield impliesthat the apertures act as half-wave resonant “slot antenna”. This allowsus to make predictions about maximum aperture sizes for a given SE: fora single aperture, SE=20 log (Ω/2d) where Ω is the wavelength at thefrequency of interest and d is the longest dimension of the aperture.

assumption may not always be accurate, but it has

being an easy design tool which is a good framework. It may be possibleto refine this formula following practical experiences with thetechnologies and construction methods used on specific products.

The resonant frequency of a slot antenna is governed by its longestdimension—its diagonal. It makes little difference how wide or narrow anaperture is, or even whether there is a line-of-sight through theaperture.

Even apertures, the thickness of a paint or oxide film, formed byoverlapping metal sheets, still radiate (leak) at their resonantfrequency just as well as if they were wide enough to poke a fingerthrough. One of the most important EMC issues is keeping the product'sinternal frequencies internal, so they don't pollute the radio spectrumexternally.

The half-wave resonance of slot antenna (expressed in the above rule ofthumb: SE=20 log(2d)) using the relationship ν=ƒλ (where ν is the speedof light: 3.10⁸ metres/sec, ƒ is the frequency in Hz, and is thewavelength in metres). We find that a narrow 430 mm long gap along thefront edge of a 19-inch rack unit's front panel will be half-waveresonant at around 350 MHz. At this frequency, our example 19″ frontpanel is no longer providing much shielding and removing it entirelymight not make much difference.

For an SE of 20 dB at 1 GHz, an aperture no larger than around 16 mm isneeded. For 40 dB this would be only 1.6 mm, requiring the gaskets toseal apertures and/or the use of the waveguide below cut-off techniquesdescribed later. An actual SE in practice will depend on internalresonances between the walls of the enclosure itself, the proximity ofcomponents and conductors to apertures (keep noisy cables such as ribboncables carrying digital busses well away from shield apertures andjoints) and the impedances of the fixings used to assemble the parts ofthe enclosure, etc.

Wherever possible, it is desirable to break all necessary or unavoidableapertures into a number of smaller ones. Unavoidably long apertures(covers, doors, etc) may need conductive gaskets or spring fingers (orother means of maintaining shield continuity). The SE of a number ofsmall identical apertures nearby each other is (roughly) proportional totheir number (SE=20 logn, where n is the number of apertures), so twoapertures will be worse by 6 dB, four by 12 dB, 8 by 18 dB, and so on.But when the wavelength at the frequency of concern starts to becomecomparable with the overall size of the array of small apertures, orwhen apertures are not near to each other (compared with thewavelength), this crude 6 dB per doubling rule breaks down because ofphase cancellation effects.

Apertures placed more than half a wavelength apart do not generallyworsen the SEs that achieves individually, but half a wavelength at 100MHz is 1.5 metres. At such low frequencies on

than this, an increased number of apertures will

enclosures SE.

Apertures don't merely behave as slot antenna. Currents flowing in ashield and forced to divert their path around an aperture will cause itto emit magnetic fields. Voltage differences across an aperture willcause the aperture to emit electric fields. The author has seen dramaticlevels of emissions at 130 MHz from a hole no more than 4 mm in diameter(intended for a click-in plastic mounting pillar) in a small PCB-mountedshield over a microcontroller.

The only really sensible way to discover the SE of any complex enclosurewith apertures is to model the structure, along with any PCBs andconductors (especially those that might be near any apertures) with a3-dimensional field solver. Software packages that can do this now havemore user-friendly interfaces and run on desktop PCs. Alternatively, theuser will be able to find a university or design consultancy that hasthe necessary software and the skills to drive it.

Since an SE will vary strongly with the method and quality of assembly,materials, and internal PCBs and cables, it is always best to allow anSE ‘safety margin’ of 20 dB. It may also be advantageous to at leastinclude design-in features that will allow improvement of the SE by atleast 20 dB if there are problems with the final design'sverification/qualification testing.

The frequency of 50 Hz is problematic, and an SE at this frequency withany reasonable thickness of ordinary metals is desirable. Specialmaterials such as Mumetal and Radiometal have very high relativepermeabilities, often in the region of 10,000. Their skin depth iscorrespondingly very small, but they are only effective up to a few tensof kHz. It is advantageous to take care not to knock items made of thesematerials, as this ruins their permeability and they have to be thrownaway or else re-annealed in a hydrogen atmosphere. These exoticmaterials are used rather like channels to divert the magnetic fieldsaway from the volume to be protected. This is a different concept tothat used by ordinary shielding.

All metals shield materials with relative permeability greater than 1can saturate in intense magnetic fields, and then don't work well asshields and often heat up. A steel or Mumetal shield box over a mainstransformer to reduce its hum fields can saturate and fail to achievethe desired effect. Often, this is all that is necessary to make the boxlarger so it does not experience such intense local fields. Anothershielding technique for low frequency shielding is active cancellation,and at least two companies have developed this technique specificallyfor stabilizing the images of CRT VDUs in environments polluted by highlevels of power frequency magnetic fields.

FIG. 1D shows that if we extend the distance that a wave leaking throughan aperture has to travel between surrounding metal walls before itreaches freedom, we can achieve respectable SEs even though theapertures may be large enough to put a fist through. This very powerfultechnique is called “waveguide below cut-off”. Honeycomb metalconstructions are really a

below cut-off stacked side-by-side, and are often

for shielded rooms, similar to high-SE enclosures. Like any aperture, awaveguide allows all its impinging fields to pass through when itsinternal diagonal (g) is half a wavelength. Therefore, the cut-offfrequency of our waveguide is given by: ƒcutoff=150,000/g (answer in MHzwhen g is in mm.) Below its cut-off frequency, a waveguide does not leaklike an ordinary aperture (as shown by FIG. 1A) and can provide a greatdeal of shielding: for ƒ<0.5 ƒcutoff SE is approximately 27 d/g where dis the distance through the waveguide the wave has to travel before itis free.

FIG. 1A shows examples of the SE achieved by six different sizes ofwaveguides below cut-off. Smaller diameter (g) results in a highercut-off frequency, with a 50 mm (2 inch) diameter achieving fullattenuation by 1 GHz. Increased depth (d) results in increased SE, withvery high values being readily achieved.

Waveguides below cut-off do not have to be made out of tubes, and can berealized using simple sheet metalwork which folds the depth (d) so asnot to increase the size of the product by much. As a technique, it isonly limited by the imagination, but it must be taken into considerationearly in a project as it is usually difficult to retro-fit to a failingproduct not intended for use in conductors should never be passedthrough waveguides below cut-off, as this compromises theireffectiveness. Waveguides below cut-off can be usefully applied toplastic shafts (e.g. control knobs) so that they do not compromise theSE where they exit an enclosure. The alternative is to use metal shaftswith a circular conductive gasket and suffer the resulting friction andwear. Waveguides below cut-off can avoid the need for continuous stripsof gasket, and/or for multiple fixings, and thus save material costs andassembly times.

Gaskets are used to prevent leaky apertures at joints, seams, doors andremovable panels. For fit-and-forget assemblies, gasket design is nottoo difficult, but doors, hatches, covers, and other removable panelscreate many problems for gaskets, as they must meet a number ofconflicting mechanical and electrical requirements, not to mentionchemical requirements(to prevent corrosion). Shielding gaskets aresometimes required to be environmental seals as well, adding to thecompromise.

FIG. 1B shows a typical gasket design for the door of an industrialcabinet, using a conductive rubber or silicone compound to provide anenvironmental seal as well as an EMC shield. Spring fingers are oftenused in such applications as well.

It is worth noting that the green/yellow wire used for safety earthingof a door or panel has no benefits for EMC above a few hundred kHz. Thismight be extended to a few MHz if a short wide earthing strap is usedinstead of a long wire.

of gasket types is available from a number of

of whom also offer customizing services. This observation reveals thatno one gasket is suitable for a wide range of applications.Considerations when designing or selecting gaskets include: (1)mechanical compliance; (2) compression set; (3) impedance over a widerange of frequencies; (4) resistance to corrosion (low galvanic EMFs inrelation to its mating materials, appropriate for the intendedenvironment); (5) the ability to withstand the expected rigors of normaluse; (6) shape and preparation of mounting surface (7) ease of assemblyand dis-assembly; and (8) environmental sealing, and smoke and firerequirements.

There are four main types of shielding gaskets: conductive polymers,conductively wrapped polymers, metal meshes and spring fingers. (1)Conductive polymers (insulating polymers with metal particles in themdouble as environmental seals, and have low compression set but needsignificant contact pressure, making them difficult to use inmanually-opened doors without lever assistance. (2) Conductively wrappedpolymers (polymer foam or tube with a conductive outer coating can bevery soft and flexible, with a low compression set. Some only need lowlevels of contact pressure. However, they may not make the bestenvironmental seals and their conductive layer may be vulnerable towear. (3) Metal meshes (random or knitted) are generally very stiff butmatch the impedance of metal enclosures better and so have better SEsthan the above types. They have poor environmental sealing performance,but some are now supplied bonded to an environmental seal, so that twotypes of gaskets may be applied in one operation. (4) Spring fingers(“finger stock”)are usually made of beryllium copper or stainless steeland can be very compliant. Their greatest use is on modules (and doors)which must be easy to manually extract (open), easy to insert (close),and which have a high level of use. Their wiping contact action helps toachieve a good bond, and their impedance match to metal enclosures isgood, but when they don't apply high pressures, maintenance may berequired (possibly a smear of petroleum jelly every few years). Springfingers are also more vulnerable to accidental damage, such as gettingcaught in a coat sleeve and bending or snapping off. The dimensions ofspring fingers and the gaps between them causes inductance, so for highfrequencies or critical use a double row may be required, such as can beseen on the doors of most EMC test chambers.

Gaskets need appropriate mechanical provisions made on the product to beeffective and easy to assemble. Gaskets simply stuck on a surface andsquashed between mating parts may not work as well as is optimal—themore their assembly screws are tightened in an effort to compress thegasket and make a good seal, the more the gaps between the fixings canbow, opening up leaky gaps. This is because of inadequate stiffness

, and it is difficult to make the mating parts rigid

a groove for the gasket to be squashed into, as shown by FIG. 1B. Thisgroove also helps correctly position and retains the gasket duringassembly.

Gasket contact areas must not be painted (unless it is with conductivepaint), and the materials used, their preparation and plating must becarefully considered from the point of view of galvanic corrosion. Allgasket details and measures must be shown on manufacturing drawings, andall proposed changes to them must be assessed for their impact onshielding and EMC. It is not uncommon, when painting work is transferredto a different supplier, for gaskets to be made useless because maskinginformation was not put on the drawings. Changes in the paintingprocesses used can also have a deleterious effect (as can differentpainting operatives) due to varying degrees of overspray into gasketmounting areas which are not masked off.

FIG. 1C shows a large aperture in the wall of the shielded enclosure,using an internal “dirty box” to control the field leakage through theaperture. The joint between the dirty box and the inside of theenclosure wall must be treated the same as any other joint in theshield.

A variety of shielded windows are available, based on two maintechnologies: thin metal films on plastic sheets and embedded metalmeshes. (1) Thin metal films on plastic sheets, usually indium-tin-oxide(ITO). At film thicknesses of 8 microns and above, optical degradationstarts to become unacceptable, and for battery-powered products, theincreased backlight power may prove too onerous. The thickness of thesefilms may be insufficient to provide good SEs below 100 MHz, (2)Embedded metal meshes, are usually made of a fine mesh of blackenedcopper wires. For the same optical degradation as a metal film, theseprovide much higher SEs, but they can suffer from Moire fringing withthe display pixels if the mesh is not sized correctly. One trick is toorient the mesh diagonally.

Honeycomb metal display screens are also available for the very highestshielding performance. These are large numbers of waveguides belowcut-off, stacked side by side, and are mostly used in security ormilitary applications. The extremely narrow viewing angle of thewaveguides means that the operator's head prevents anyone else fromsneaking a look at their displays.

The mesh size must be small enough not to reduce the enclosure's SE toomuch, The SE of a number of small identical apertures near to each otheris (roughly) proportional to their number, n, (□SE=20 logn), so twoapertures will make SE worse by 6 dB, four by 12 dB, 8 by 18 dB, and soon. For a large number of small apertures typical of a ventilationgrille, mesh size will be considerably smaller than one aperture on itsown would need to be for the same SE. At higher frequencies where thesize of the ventilation aperture exceeds one-quart

, this crude “6 dB per doubling” formula can

engineering, but no simple rule or thumb exists for this situation.

Waveguides below cut-off allow high air flow rates with high values ofSE. Honeycomb metal ventilation shields (consisting of many long narrowhexagonal tubes bonded side-by-side) have been used for this purpose formany years. It is believed that at least one manufacturer of highlyshielded 19″ rack cabinets claims to use waveguide below cut-offshielding for the top and bottom ventilation apertures that use ordinarysheet metalwork techniques.

The design of shielding for ventilation apertures can be complicated bythe need to clean the shield of the dirt deposited on it from the air.Careful air filter design can allow ventilation shields to be welded orotherwise permanently fixed in place.

Plastic enclosures are often used for a pleasing feel and appearance,but can be difficult to shield. Coating the inside of the plasticenclosure with conductive materials such as metal particles in a binder(conductive paint), or with actual metal (plating), is technicallydemanding and requires attention to detail during the design of themould tooling if it is to stand a chance of working.

It is often found, when it is discovered that shielding is necessary,that the design of the plastic enclosure does not permit the required SEto be achieved by coating its inner surfaces. The weak points areusually the seams between the plastic parts; they often cannot ensure aleak-tight fit, and usually cannot easily be gasketted. Expensive newmould tools are often needed, with consequent delays to marketintroduction and to the start of income generation from the new product.

Whenever a plastic case is required for a new product, it is financiallyvital that consideration be given to achieving the necessary SE rightfrom the start of the design process.

Paint or plating on plastic can never be very thick, so the number ofskin-depths achieved can be quite small. Some clever coatings usingnickel and other metals have been developed to take advantage ofnickel's reasonably high permeability in order to reduce skin depth andachieve better SE.

Other practical problems with painting and plating include making themstick to the plastic substrate over the life of the product in itsintended environment. This is not easy to do without expert knowledge ofthe materials and processes. Conductive paint or plating flaking offinside a product can do a lot more than compromise EMC—it can short outconductors, causing unreliable operation and risk fires andelectrocution. Painting and plating plastics must be done by expertswith long experience in that specialized field.

with painting or plating plastics is voltage

products (double insulated), adding a conductive layer inside theplastic cases can reduce creepage and clearance distances and compromiseelectrical safety. Also, for any plastic-cased product, adding aconductive layer to the internal surface of the case can encouragepersonnel electrostatic discharge (ESD) through seams and joints,possibly replacing a problem of radiated interference with the problemof susceptibility to ESD. For commercial reasons, it is important thatcareful design of the plastic enclosure occurs from the beginning of thedesign process if there is any possibility that shielding mighteventually be required.

Some companies box cleverly (pun intended) by using thin andunattractive low-cost metal shields on printed circuit boards or aroundassemblies, making it unnecessary for their pretty plastic case to dodouble duty as a shield. This can save a great deal of cost andheadache, but must be considered from the start of a project or elsethere will be no room available (or the wrong type of room) to fit suchinternal metalwork.

Volume-conductive plastics or resins generally use distributedconductive particles or threads in an insulating binder which providesmechanical strength. Sometimes these suffer from forming a “skin” of thebasic plastic or resin, making it difficult to achieve good RF bondswithout helicoil inserts or similar means. These insulating skins makeit difficult to prevent long apertures which are created at the joints,and also make it difficult to provide good bonds to the bodies ofconnectors, glands, and filters. Problems with the consistency of mixingconductive particles and polymers can make enclosures weak in some areasand lacking in shielding in others.

Materials based on carbon fibres (which are themselves conductive) andself-conductive polymers are starting to become available, but they donot have the high conductivity of metal and so do not give as good an SEfor a given thickness. The screens and connectors (or glands) of allscreened cables that penetrate a shielded enclosure, and their 360°bonding, are as vital a part of any “Faraday Cage” as the enclosuremetalwork itself. The thoughtful assembly and installation of filtersfor unshielded external cables is also vital to achieve a good SE. Referto the draft IEC 1000-5-6 (95/210789 DC from BSI) for best practices inindustrial cabinet shielding (and filtering). Refer to BS IEC61000-5-2:1998 for best practices in cabling (and earthing).

Returning to our original theme of applying shielding at as low a levelof assembly as possible to save costs, we should consider the issues ofshielding at the level of the PCB. The ideal PCB-level shield is atotally enclosing metal box with shielded connectors and feedthroughfilters mounted in its walls, which is in fact just a miniature versionof a product-level shielded enclosure as described above. The result isoften called a

extremely high SEs, and is very often used in

microwave worlds.

Lower cost PCB shields are possible, although their SE is not usually asgood as a well-designed module. It all depends upon a ground plane in aPCB used to provide one side of the shield, so that a simple five-sidedbox can be assembled on the PCB like any other component. Soldering thisfive-sided box to the ground plane at a number of points around itscircumference creates a “Faraday cage” around the desired area ofcircuitry. A variety of standard five-sided PCB-mounted shielding boxesare readily available, and companies who specialize in this kind ofprecision metalwork often make custom designs. Boxes are available withsnap-on lids so that adjustments may easily be made, test pointsaccessed, or chips replaced, with the lid off. Such removable lids areusually fitted with spring-fingers all around their circumference toachieve a good SE when they are snapped in place.

Weak points in this method of shielding are obviously the differentvariations of apertures such as the following: the apertures created bythe gaps between the ground-plane soldered connections; any apertures inthe ground plane (for example clearances around through-leads and viaholes); and any other apertures in the five-sided box (for exampleventilation, access to adjustable components, displays, etc.)Seam-soldering the edges of a five-sided box to a component-side groundplane can remove one set of apertures, at the cost of a time-consumingmanual operation.

For the lowest cost, we want to bring all our signals and power into theshielded area of our PCB as tracks, avoiding wires and cables. Thismeans we need to use the PCB equivalents of bulkhead-mounting shieldedconnectors and bulkhead-mounting filters.

The PCB track equivalent of a shielded cable is a track run between twoground planes, often called a “stripline.” Sometimes guard tracks arerun on both sides of this “shielded track” on the same copper layer.These guard tracks have very frequently via holes bonding them to thetop and bottom ground planes. The number of via holes per inch is thelimiting factor here, as the gaps between them act as shield apertures(the guard tracks have too much inductance on their own to provide agood SE at high-frequencies). Since the dielectric constant of the PCBmaterial is roughly four times that of air, when FIGS. 1A-1E are used todetermine via spacing, their frequency axes should be divided by two(the square root of the PCB's dielectric constant). Some designers don'tbother with the guard tracks and just use via holes to “channel” thetrack in question. It may be a good idea to randomly vary the spacingsof such rows of via holes around the desired spacing in order to helpavoid resonances.

enter an area of circuitry enclosed by a shielded

sufficient that their upper and lower ground planes (and any guardtracks) are bonded to the screening can's soldered joints on both sidesclose to the stripline.

The track which only has a single ground plane layer parallel, the otherside being exposed to the air, is said to be of “microstrip”construction. When a microstrip enters a shielded PCB box, it willsuffer an impedance discontinuity due to the wall of the box. If thewavelength of the highest frequency component of the signals in themicrostrip is greater than 100 times the thickness of the box wall (orthe width of box mounting flange), the discontinuity may be too brief toregister. But where this is not the case, some degradation inperformance may occur and such signals are best routed using striplines.

All unshielded tracks must be filtered as they enter a shielded PCBarea, It is often possible to get valuable improvements using PCBshielding without such filtering, but this is difficult to predict.Therefore, filtering should always be designed-in (at least onprototypes, only being removed from the PCB layout after successful EMCtesting).

The best filters are feedthrough types, but to save cost it isadvantageous to avoid wired types. Leaded PCB-mounting types areavailable and can be soldered to a PCB in the usual manner. Then theleaded PCB mount is hand-soldered to the wall of the screening box whenit is fitted at a later stage. Quicker assembly can be achieved bysoldering the central contact of the filter to the underlying groundplane, making sure that solder joints between the shielding box and thesame ground plane layer are close by on both sides. This latterconstruction also suits surface-mounted “feed-through” filters, furtherreducing assembly costs.

But feed-through filters, even surface mounted types, are still moreexpensive than simple ferrite beads or capacitors. To allow the mostcost-effective filters to be found during development EMC testing,whilst also minimizing delay and avoiding PCB layout iterations,multipurpose pad patterns can easily be created to take any of thefollowing filter configurations: (1) zero-ohm link (no filtering, oftenused as the starting point when EMC testing a new design); (2) aresistor or ferrite bead in series with the signal; (3) a capacitor tothe ground plane; (4) common-mode chokes; (5) resistor/ferrite/capacitorcombinations (tee, LC, etc. see Part 3 of this series for more details);(6) feed-through capacitor (i.e. centre-pin grounded, not trulyfeed-through) and; (7) feedthrough filter (tee, LC, etc., center-pingrounded, not truly feedthrough). Multipurpose padding also means theinvention not restricted to proprietary filters and be created to bestsuit the requirements of the circuit (and the product as a whole) at thelowest cost.

In finding EMI/EMC solutions, the existing technology is inelegant andcumbersome. For example, the prior art uses spoons, which are theselittle projections with dimples

out; so that they go into compression and go

over the other so that they go together and they have to make physicalcontact. These structures bend and when one of them bends at a plane andthey don't make contact anymore, they lose their electrical conduct.Then the prior art starts to have EMI leaks. They become tolerancenightmares and they're expensive. In addition, prior art manufacturingtechniques designed to counter these problems requires forming theenclosure so that it has to have a tongue and groove or otherprohibitive solutions.

SUMMARY OF THE INVENTION

The present invention removes the need for the single most expensive andleast reliable aspect of the electromechanical packaging, which is theEMI gasket. The solution(s) provided by the present invention willeliminate the need for gaskets in a great number of applications, aswell as “spoons” and other similarly troublesome structures in the PCChassis. The present invention provides a configuration of a hard-drivecovering system and method for manufacturing in which a polymerincluding an electromagnetic interference shielding (EMI shielding) isconfigured such that shielding gaskets may be reduced or eliminatedcompletely. Patterned “cuts” into one or more sides of a disk-driveholder made of Premier® or another EMI shielding polymer materialprovides sufficient EMI shielding, having the result that shieldinggaskets are not needed. In an alternate configuration of the invention,a computer box is provided with an inexpensive shielding solution

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate various prior art electromagnetic interferenceshielding principles;

FIG. 2 illustrates a sample pattern in an enclosure as may beimplemented in the invention that embodies the principle of “effectivelength;” in the two-dimensional EMI shielding solution;

FIG. 3 illustrates a side of a computer enclosure in another embodimentof the invention, or three cuts in the “four-cut” or TORTURED PATH™solution;

FIG. 4A illustrates a two-dimensional embodiment of a computer enclosurefrom a top-side view;

FIG. 4B is a close up the 2-D EMI shielding principle in a computerenclosure;

FIG. 4C is a side view of the two-dimensional EMI shielding enclosure;

FIG. 4D is a front view of the two-dimensional EMI shielding enclosure;

FIG. 5A illustrates a first embodiment of the present invention as itimplements three-dimensional shapes;

the first embodiment of the present invention

angle,

FIG. 6 illustrates the planes of the shapes in the five-sided box in afirst embodiment of the invention from a frontal view;

FIG. 7 illustrates the planes of the shapes stamped into the flange ofthe first embodiment of the invention;

FIG. 8A illustrates sample details of the “female” partial spheres;

FIG. 8B illustrates a close-up view of the “male” partial spheres;

FIG. 9A illustrates the principle of the EMI shielding in a samplethree-dimensional shape of the inventive enclosure;

FIG. 9B illustrates the principle of the EMI shielding from in anassembled enclosure;

FIG. 10 illustrates further operations of the invention in providing EMIshielding;

FIG. 11A illustrates the interior of the three-dimensional embodiment ofthe invention in the box component;

FIG. 11B illustrates a top-down view of the invention in the lidcomponent;

FIG. 11C shows a side view of the box component in a first embodiment;

FIG. 12 illustrates an alternate embodiment of the invention using asingle row of cubic shapes;

FIG. 13 shows a double row of cubic shapes in another alternateembodiment; and

FIG. 14 shows a hybrid configuration using both two-dimensional andthree-dimensional shielding structures.

DETAILED DESCRIPTION OF THE DRAWINGS

With the “shell” or “scallop” embodiment of the invention, as shown inFIGS. 5A and 5B, the three-dimensional patterns are formed or otherwiseconfigured such that they are generally going the inside periphery ofthe edges, and the two parts FSE and FL come together and the“sinusoids” meet. All that is necessary for the implementation of thethree-dimensional implementation of the invention is to “cut” or stampthe edge of the metal and make the same cut and they come together witha “30 gap” or something similar. The advantages of the primaryembodiment of the invention include, inter alia, the fact that theredoes not need to be any contact and therefore no degradation over time(see FIG. 9B). The parts FSE and FL don't have to make physical contact.Further advantages include that there are no tolerances to consider andthere is nothing to deform.

to FIG. 5A, a first embodiment of the

solutions for electronics enclosures is shown from a side-angled view.The first embodiment takes advantage of the manufacturing ease of usinga two-part enclosure including a five-sided enclosure FSE with aninterior volume IN for housing electronics and a flange FL, which fitsinto the five-sided enclosure upon completion.

Referring again to FIGS. 5A-5B shows a top view of a three-dimensionalbox in a two-part embodiment of the three-dimensional solution (Athree-part embodiment of the invention is discussed briefly below). Inthis particular embodiment, either the box or the flange could be moldedor cast, and thus “three-dimensional tortured path” or a TORTURECHAMBER™ is illustrated. In general, the electromagnetic interferencecannot get in or out of the electronic enclosure. In the preferredembodiment shown in FIGS. 5A and 5B, there is a (periodic) quartersphere with a half cylinder-type shape IP, although, as can beappreciated by those skilled in the art, many other types of shapeswould be sufficient for providing the necessary shielding, and some arebriefly discussed below. In the illustration, the female threedimensional shapes FP in the “lid” or flange FL or mate with the maleprotrusions IP along the perimeter of the lid at the lid-to-boxinterface OE, which is generally the XY plane formed at the seam of thejunction between the lid and the box (not shown), labeled as planeXY(#A).

Even though there can be adequate spacing between the box FSE and thelid/flange FL, the shielding is provided well inside the allowable forthe frequency that are generally desired for shielding.

As shown in FIGS. 5A and B, the three-dimensional EMI-shielding solutionincludes an interior pattern IP of three-dimensional shapes which arestamped, cut, molded, extruded or otherwise configured into thefive-sided enclosure FSE around the perimeter of the top or open edgeOE. FIGS. 5A and 5B show the interior pattern IP as being semi-sphericaland “male” or protruding into the interior volume IN, however, in otherembodiments the shapes could be reversed or “female” without necessarilydeparting from the spirit of the invention. FIGS. 5A and 5B show thatthe flange FL also includes a pattern that is “complementary” to eachother such that the box and the phalange will seamlessly fit as well asprovide sufficient EMI shielding.

The half-wave resonance of slot antenna, expressed in the above rule ofthumb, is the basis for the solid line in FIG. 1D (and for therule-of-thumb of FIG. 1E) using the relationship: SE=20 log (λ/2d).Therefore the degradation associated with a multiple hole pattern isgiven by: SE reduction=10 log (N), where N=the # of holes in thepattern. Using the relationship: f•λ=c, where is c the speed of light:3×10̂8 m/sec, the frequency in Hz, and λ is the wavelength in meters,where: f=the frequency of the wave λ=the wavelength, c=the speed oflight.

be use of conductive materials to reduce EMI by

absorption. Shielding electronic products successfully from EMI is acomplex problem with three essential ingredients: a source ofinterference, a receptor of interference, and a path connecting thesource to the receptor. If any of these three ingredients is missing,there is no interference problem. Interference takes many forms such asdistortion on a television, disrupted/lost data on a computer, or“crackling” on a radio broadcast. The same equipment may be a source ofinterference in one situation and a receptor in another.

Currently, the FCC regulates EMI emissions between 30 MHz and 2 GHz, butdoes not specify immunity to external interference. As devicefrequencies increase (applications over 10 GHz are becoming common),their wavelengths decrease proportionally, meaning that EMI canescape/enter very small openings (for example, at a frequency of 1 GHz,an opening must be less than ½ inch). The trend toward higherfrequencies therefore is helping drive the need for more EMI shielding.As a reference point, computer processors operate in excess of 250 MHzand some newer portable phones operate at 900 MHz.

Metals (inherently conductive) traditionally have been the material ofchoice for EMI shielding. In recent years, there has been a tremendoussurge in plastic resins (with conductive coatings or fibers) replacingmetals due to the many benefits of plastics. Even though plastics areinherently transparent to electromagnetic radiation, advances incoatings and fibers have allowed design engineers to consider the meritsof plastics.

As a specific example, considering the FCC regulation to shield up to 2GHz, a typical maximum clock speed in many of the controllers in theenterprise networks would be 400 MHz. If you consider the 2 GHz value asthe maximum frequency of interest, then at 400 MHz, the user will shieldup to and including the 5th harmonic of a 400 MHz signal . . . i.e. 400MHz*5=2 GHz (shielding to the 5th harmonic of maximum clock speed of 400MHz).

) To determine the wavelength at 2 GHz, utilize equation C, above:f•λ=c, λ=c/f λ=(3×108)/(2*109 λ=0.15 meters (at 2 GHz). Terms A & B areof interest regarding the determination of a longest possible slotlength λ/2=0.075m or 75 mm. It is recommended that the apertures be keptto a range of approximately λ/20 to λ/50, therefore for 2 GHz, theapertures should be in the range of: λ/20=0.0075 meters or 7.5 mmmaximum@2 GHz; λ/50=0.003 meters or 3.0 mm minimum@2 GHz.

Looking to equation from above, the shielding effectiveness for 1 holeof maximum length “X”: SE=20 log (λ/2d) (there is no minimum—the smallerthe better—this equation is used as a practical value for packaging.) @3mm - - - >SE=20 log (0.15/(2·0.003))=20 log (25)=28 dB′@7.5mm - - - >SE=20 log (0.15/(2·0.0075))=20 log (10) =20 dB.

in a standard application there are multiple holes

perfed 0.060″ thick steel faceplate SE reduction=10 log (N) has a holepattern comprised of 100 holes and an SE reduction=10 log (N)=10 log(100)=20. The result is the reduction of the shielding to zero in thecase of the 7.5 mm holes and the reduction of the shielding to 8 dB inthe case of the 3 mm holes.

This is where the restrictive nature of EMI emerges and the interplaybetween getting cooling air in without letting magnetic interference outbecomes more significant. One of the principles upon which the inventiontakes advantage of is illustrated by FIG. 1A.

It is recommended that most packaging applications provide ˜15 dB ofshielding at the enclosure level. As is evident from the aboveinformation, this is far from easy to accomplish without an advance inthe technology. It should be noted that the degradation described abovedoes not even consider all the losses at seams where the gaskets areactually used. This is only the perf for airflow.

In the two-dimensional solution for EMI shielding is shown forenclosures that are generally in the shape of boxes and other types ofcabinets for computers and other electronic components that requireEMI/EMC shielding. Referring to FIG. 1A, a principle the wall ofenclosure is shown which is the wall of a shielded enclosure made of aconductive material, with the greater sizes of apertures causing agreater amount leakage of the electromagnetic fields. In an embodimentof the invention known by the trade name of “TORTURED PATH™” theimprovement reduces the size of apertures by strategically cutting,forming, molding, extruding, stamping and forming any manufacturingmethod which utilizes an electromagnetically conductive material inbasically any application.

The present invention provides a less expensive EMI shielding solutionthan the way the current technology is implemented. This can beaccomplished in various embodiments of the invention implemented “two”dimensions (namely two-dimensional considerations since nothingliterally takes place in only two dimension) with sheet metal or flatextruded cut or stamped materials. The material could be cast, again,with a thin sheet metal—assuming that the structures cast, cut, orextruded are thin relative to the overall dimensions, considering thatthe so-called two-dimensional considerations have finite thickness. Asthe manufacturing goes into a molding process or casting, it creates amore even three-dimensional shape or forms metal out of the 2D planesand uses drying techniques to create overlaps and further “torture thepath.” Thus, a goal of this particular embodiment of the invention is tocreate small apertures. More particularly, the goal of this embodimentis to create apertures that are not only small but force theelectromagnetic noise to change directions or to go through aperturesthat are small and make the path difficult for the EMI to find its wayout (thus, the “tortured path”). This, of course,

susceptibility of the electronics inside the enclosure

electromagnetic interference from the outside as well. EMI,electromagnetic inference generally refers to what is projected outwardsto the world and how it might interfere with other devices. However, forthe purposes of this disclosure, the expression “EMI” also includesshielding from any devices that are external to the user and that areradiating electromagnetic fields which will cause interference on theproduct and this is where the user would be susceptible to EMI.

Wave guides are discussed above in FIGS. 1A-E, in which the depth of achannel or depth of an aperture causes an increased difficulty for theelectromagnetic wave to get out for any given aperture size. TheTORTURED PATH™ invention is implemented in a mold or a cast to create athree-dimensional pathway that does not allow EMI to escape or enter theenclosure, which may include a wave guide effect. But, again, thepreferred and conceptually most effective tortured path for the EMI is asinusoidal saw-tooth square wave, as shown in FIG. 2, but also may beany kind of irregular shape, as shown in FIG. 3 whether the pattern isperiodic, periodic and changing, or constantly changing in shape.However, the invention requires that the pattern not allow for themaximum aperture size to be sufficient for the electromagnetic waves totraverse through the material, whether it is inward or outward. Thisprinciple of the invention is shown in FIG. 2 as “effective lengthmatters.”

Prior art illustration FIG. 1B shows a phalange which is used like achannel with a gasket. This channel is in the base of a box or anenclosure and then filled with a gasket—a circular gasket in this caseis a very common approach. Then a lid is applied that forces that gasketto deform and it will partially conform into the channel thereby forminga seal. If this “tortured path” concept is considered, as is seen in theillustrations of the present invention discussed below where athree-dimensional example is shown, the gasket can be partially or fullyavoided by molding, casting, or machining a shape in which the top fitsinto the bottom. However, the fitting procedure generally includes morethan the simple wave guide effect as shown in the prior art.

The invention uses the so-called “tortured path” feature in concert withor uniquely to create a shape that reduces the aperture size byimproving the configuration of the metals fit together in particularconfigurations that provide the desired EMI/EMC shielding. In apreferred embodiment, shown in FIGS. 5A-14, discussed below, aparticular configuration uses sinusoidal three-dimensional scallops thatare shaped in two dimensions and then shaped again into the orthogonalplanes. The orthogonal planes have sweeping shapes that force the EMI totraverse through narrowed apertures for the use of shaping. Therefore,since the material around the chassis, (which must be electromagnetic

use the EMI/EMC shielding won't work if it's not)

be in contact with an electromagnetic conductive material. Using aconductive material with this configuration means that the wave istrying to get through an aperture that is too small for it to emit orreceive waves of a given frequency. Changing the shape of the cuts, itis possible to do that again in concert with either the wave guide, aseam that is a tongue and groove or a stamp so that you have aninterlocking or with metal that's hem-on-hem. But instead of just havingthe hem interlocking, this “tortured path” shape is created and fitsthem into each other with male and female opposing images with a gap.This does not require the invention to have tight tolerances becausethis gap can be relatively small compared to the allowable aperturesize, but very large compared to the allowable tolerances. Because ofthis particular feature of the present invention, 100% reliability onassembly is possible.

Additionally, 100% reliability is possible in the performance withparticular embodiments of the invention, because the medium is notvulnerable to compression or degradation over time. Additionally, thereis not any material used as a gasket that will be ripped off andsheared, nor is there a gasket that will plastically deform. Berylliumcopper, for example, can plastically deform. Additionally, any metalgasket, finger gaskets or finger stock can either deform throughimproper design or improper handling, whether that be in shipping orother in situations. Instead, by creating cuts or through two orthree-dimensional cuts that control the EMI as a way to control theaperture size, there is nothing to deform. Furthermore, in the presentinvention, there is no requirement for physical contact, therefore thereare no tolerance issues, deforming issues, no degradation over time andno environmental impact. There are no loose structures added. Theinvention provides an extremely cost effective EMI shielding solutionbecause there are no added parts, no fasteners and no welds. Free-platedmaterial may be used everywhere which are formed in the case of sheetmetal, stamp and form and/or a few rivets which do not depend on contactwhich have no degradation over time and no environmental impact.

A prior art panel mount FIG. 1C, is shown as seen through a box orthrough the face may be adapted to particular embodiments of the presentinvention. With the preferred embodiment of the tortured path invention,this may be generally efficiently created, for example, with a panelmounted meter as shown. On the backside of the phalange that mates tothe chassis, which adapts the prior art to implement the presentinvention by adding the TORTURED PATH™ shape on a plate behind thephalange. In the case of sheet metal, another piece of sheet metal canbe used behind the flushed phalange that would be a ninety-degreerectangular shape on the front, which would also allow for aestheticallyappropriate or pleasing patterns. Behind, the cut materials, sinusoidal,saw tooth, square waves, would fit into an aperture, again, of themating shape with appropriate tolerances

20-30 thousandths of an inch in a gap that follow

Then, the negative of that shape would follow around and would justoverlay on top of one another into the same plane. There would be justthis gap of whatever shape was selected, reducing the effective lengthall the way around the perimeter and containing the EMI and providesufficient shielding.

Referring now to FIG. 1E a diagram of the relationship between frequencyand gap size is shown. As the frequency of the electromagnetic noisegoes up, the allowable aperture size goes down in order to have adequateshielding. Obviously as the frequency goes up, it is necessary to havesmaller and smaller gaps. In this way, tortured path, if the sinusoidwas used, for example, the wave length may be shortened and theamplitude lowered in order to create a gap appropriate for a givenfrequency. It works well because it is below the allowable halfwavelength of the range which is typical for shielding applications. Forexample, at a range of land over 50 to lambda which is over 20wavelengths at a given frequency, divided by 20 to 50 in that range. Asan example, the FCC regulates up to two gigahertz and in that range,it's lambda over 50 equals three millimeters, land being equal to 0.15meters or 150 millimeters. Divided by 50, it's three millimeters anddivided by 20 is 7½ millimeters. Even in the case of sheet metal whereyou have bends and other abnormalities of shape, it is still easy tomanage all tolerances of bends with a 30,000^(th) gap, which would givebasically 100% reliability on assembly but only be 30,000^(th) wide.Additionally, if such a situation is compared to three-quarters of onemillimeter, such that there is a wave that's three or four times thissize, it still stays within the three-millimeter range at four timesthat size. Such a gap would not allow the adjacent peaks or valleys ofthat wave to be seen, so the effective length would basically be thetraverse; the traverse would go not quite from peak to peak but partwayfrom the peak to part way to the valley, sort of down the transitionfrom peak to valley in the wave and still well within the threemillimeter requirements.

Very short wavelengths or very small aperture sizes are allowed in thisway but do not require anything other than a stamp and a form. In a caseof a mold, it is possible to run that much tighter. It may be narrowerthan 30,000, in sheet metal. That's very generous and it makes an almostperfect assembly. It is possible to reduce it to 10 or 15 thousandthsand there would be no issue. This remains true if all of the cuts areretained so that they're not visible and, if it's not exactly the maleand female, which don't fit exactly following each other, as long asthey stay within that gap, it may be slightly irregular. For example,one peak might be a little close to the valley, but it won't causeinterference and, perhaps, it could even cause an intermittent contactwhich might enhance the electro-conductivity.

to the shielding effectiveness versus the

if you look at a 10 millimeter gap, for example, it's showing that 20decibels of shielding with a 10 millimeter gap at one gigahertz arepossible, approximately. For most electromagnetic packages, in the caseof a sheet metal enclosure, it is rare that a sheet metal enclosure willproduce more than 20 DB of shielding the enclosure. In this one example,a 10-millimeter slot will provide 20 DB of shielding at one gigahertz.In this case, the sign waves easily constrain the gap to be anywherehalf of that size. At half that size, at one gigahertz, based on thischart, it might be up to about 35 DB, which is significantly above theshielding of any normal chassis. Now, of course, that's based on oneaperture. Therefore, it may be necessary to degrade that by the 10 lawagain, where N is the number of total apertures. But it takes 100apertures—the 10 law again—to have a 20 DB degradation in shielding. Sowith a five-millimeter slot, which we could easily provide, onegigahertz, would be about 30 DB of shielding. So even with 100 of those,there would still have 10 DB of shielding; there are many enclosuresthat exist that don't have far in excess 10 DB of shielding. Certainly,in consumer desktop PCs that is what would be expected more often thannot in the chassis. So, again this invention proposes a solution with nogaskets, no screws, no fasteners, and just a few rivets and withbasically no degradation of performance over time and with nocompression set, only a gap.

Referring now to FIG. 2, the first model of the EMI shielding principlein a two-dimensional application of the invention as shown in “effectivelength,” a model is shown where the LSTD is the old standard length of aslot. If it was a straight slot, that gap would be, as shown, comparedto length of the EMI-shielding path, which is the longest straight linedistance that the electromagnetic interference can “see” through thesinusoid. The length standard and the strength slot would be somewherein the order of eight to 10 times the length of the EMI shielding path.FIG. 3A shows a couple examples of the different types of shapes. Yousee a triangular sawtooth-type cut configuration. Again, just do notallow the wave to be able to see by the peaks so it will look for thestraightest line it can find. So it's just tortured in that it cannotsee around the corners. And then you see a square wave and then you seea very odd bent paperclip-looking shape wave, a cut. You can use any cutyou can imagine. What you're trying to do is reduce the effective lengthof any slot that can be used as antenna by the electromagneticinterference. So this can be used around IO [sounds like] devices. Thiscan be used in sheet metal. This can used in extruded cuts, molded,casted cuts in any shape, whether it's used in sort of a modular into achassis, around the back phalange of a model, whether it's around theinput/output devices, in any manufacturing method, anyelectromagnetically conducted materials for which EMI needs to becontained. Torture the path—EMI, reduce the effective length bystrategic cut shapes or molds or extrude shapes and, in addition, gointo a three

and overlapping, again, torturing the path. In

together a wave-guided effect, but, again, the tortured path is theessence of this concept and it can easily be brought to bear withcomplementary forming techniques or molding techniques that don't addcosts, or increase manufacturing complexity.

FIG. 3A illustrates a two-dimensional EMI shielding embodiment of theinvention. Three cuts are shown in various shapes in the illustration,and four are used in a first type of the alternate embodiment. However,the cuts may all be of one type of cut, in appropriate patterns, such assinusoidal, square wave, and certain so-called Brownian-motion typecuts. The two-dimensional EMI shielding solution provides a potentiallycomplete EMI shielding solution in alternate embodiments as long as thefour lines are placed to prevent any “snaking” of the sinusoidal wavepropagation WP. FIG. 3A shows a couple examples of the different typesof shapes. A triangular saw tooth-type cut configuration is shown TST.Again, the wave(s) are not able to seen by the peaks so it will look forthe straightest line it can find. So it's just “tortured” in that itcannot “see” around the corners. The square wave SW is then seen andthen a very odd bent paperclip-looking shape wave MWC cut is also shown.As can be appreciated, other types of cuts can be used. The goal is totry to reduce the “effective length” of any slot that can be used asantenna by the electromagnetic interference. So these cuts can be usedaround I/O devices. This can be used in sheet metal as well as can beused in extruded, molded or casted cuts in any shape. The invention maybe applied, such that it is used in a modular into a chassis or aroundthe back phalange of a model. It may be used around the input/outputdevices, in any manufacturing method or in any electromagneticallyconducted materials for which EMI needs to be contained. The TORTUREDPATH™ solution reduces the effective length by strategic cuts, shapes ormolds or extruded shapes and, in addition, goes three-dimensionalthrough drawing and overlapping, again, torturing the path. Evenbringing together a wave-guided effect. The TORTURED PATH™ is theessence of the invention and it can efficiently be implemented in thepresent invention with complementary forming techniques or moldingtechniques that don't require additional costs.

Illustrating how effective this principle is and how versatile themanufacturing applications are, FIG. 3B shows that the sides of analternately cut computer enclosure can have any number of non-periodicpatterns NPP along the seams of the enclosure ENC to create the improvedEMI shielding.

Implementing the “effective length” principle to make a two-dimensionalEMI shielding solution, is a stamp and then a subsequent form or stampwhich is brings the male and female image of these two slots together.This is achieved alternately with a small width and large width so thesmaller male fits inside the larger female, back and forth, whether

square wave, sign wave or some intermittent

other shapes. As shown, it is possible to reduce that effective lengtheconomically and efficiently at virtually no cost.

FIGS. 4A-4D illustrate an embodiment of the two-dimensional EMIshielding aspects as were introduced in U.S. Patent Publication No.2005-205209 (U.S. application Ser. No. 11/012,896) and U.S. applicationSer. No. 11/080,385, which are all hereby incorporated by reference, forall purposes. FIG. 4A is a top angle view of a first example orembodiment, a three-sided and three-sided bar where the one three-sidedfitting is down over the other and it comes straight down from the top.Then, the EMI/EMC is just deflected in front and back in order toovercome any interference between the sign waves. So it is possible tobring the two U-sections together, having The TORTURED PATH™ seamrunning along six different edges to bring the two three-sided boxes orsections together.

FIGS. 4A and 4B illustrate a sample of a two-dimensional “generic”“one-hit” solution embodiment of the invention for PC enclosures andother “boxes” that provide a great deal of advantage in themanufacturing process over current methodologies.

Other embodiments of the invention can include end-use, assembly andother manufacturing considerations such as using the “three-and-three”configuration (not shown). a 4-side component of model #3 of a preferredembodiment of the invention which can be used in a “4×1×1” or “4×2”. The4-sided component is generally manufactured with the sinusoidal patternon each of the edges, but can include other patterns as discussed inFIG. 3. In the two-dimensional EMI shielding embodiment, the phalangesall come down from the top to the sides and back. So when the lid isoff, there may be a wide-open exposure to fully populate the inside ofthe box without any interference, with none of the top view lookingdownwards covered by any material whatsoever. There is therefore fullaccess to the box. In addition, this could also be done as a four sideand a two side where the top was included as part of the whole front.This could also be done in a two-part assembly instead of three. In thecase of a “5×1 type” of the embodiment, this configuration is verystraightforward. This embodiment may also be implemented in it is atwo-part assembly and a 3×3 channel box which is also a two-partassembly.

In most embodiments of the two-dimensional EMI shielding computerenclosure applications of the invention, the invention requires a simplestamp or cut and form fabrication in sheet metal (for that particularembodiment). The invention is less expensive than “spoons” and does notrequire physical contact which therefore provides greater reliability.

to FIG. 4A, a model of the preferred

computer enclosure configuration is shown. An example of a one RU 17½inch wide chassis, approximately 11½ inches deep, which would representa classic kind of configuration for a standard 19 inch rack, 17½ incheswide allowable and one RU in height. The TORTURED PATH™ seams run alongthe sides. This is a front isometric view, looking at the frontright-hand corner in the foreground. The seams would run along the topof both sides, the top of the back, down both back vertical corners nearthe corner, but on the backside. All four sides are around thefaceplate, but none show on the front section at all. So aesthetically,no apertures are in the front, although aesthetically these could beenhancing at some point and that's something that may be discussed.

Such apertures could be used ergonomically, aesthetically or they couldbe used to cut the logo of a company, if done properly. They could alsobe used for implementing TORTURED PATH™ shapes to create air inflow in amanner which would help to control the EMI. So in the particularenclosure illustrated in FIGS. 4A and 4B, there is a four-sided basewhere in the flat pattern configuration, it would be a four-sided box.It would have a TORTURED PATH™ shape around the perimeter of all foursides. In between the back and the two side sections, which could bephalanged over, it would fold up and phalange the sides into the backpanel. Fastening requires only a rivet is in each, and optionally arivet in each of the back corners. Thus, the process would be stamp,form, and two rivets for the total assembly and no gaskets, no welds, noscrews, which could all be done in pre-plate material.

FIG. 4A also illustrates the preferred embodiment with a separate lidthat comes down that goes over the back and both sides, again, with theTORTURED PATH™ assembly. These particular configurations allow for easyaddition of joining tabs in order to rivet the top to the base or it canuse a counter-sunk screw, avoiding “intermittency” problems from astandpoint of electromagnetics which could also augment theelectromagnetic advantages of the present invention. This is contrast toa standard enclosure situation with a straight slot where it's necessaryto have the screws at the same spacing as the maximal allowableaperture. With this configuration of the present invention, the spacingmight augment the TORTURED PATH™ EMI/EMC shielding solution and also beused for structural integrity and/or be just for the enclosure tomaintain the enclosure.

In the configurations shown in FIGS. 4A and B, the nose or the frontfaceplate goes over all four sides and can be tapped and can control it.In this way, it is possible to actually put the lid down in aconfiguration where it is actually hooked rotating, in a tongue andgroove kind of hook. It is possible to bring it down and capture all theassembly strictly with the faceplate. Basically, there would then be astamp, a form, a friction fit and then just the lid

The sides and the base would be captured by the

this way, the entire assembly is brought together. While such a processmay not provide all the structural integrity that was needed for allend-uses, in many cases it would certainly be adequate. There are manyconfigurations for which this could be done. The EMI can be containedand basically element fasteners eliminate welds and gaskets at a verylow cost and additionally provide thermal enhancement. So it wouldtherefore be additionally cost reductive and, thermally enhanced becauseof the ability to now open up more apertures and would beenvironmentally friendly, without any addition there. One hundredpercent of this is assemble-able and 100% reliable with no degradationover time. There's a simple captive. It could be quarter term, but inthis case, a simple captive—spring loaded screw that can be taken into apem-nut on the back of a phalange is pivoted off of the side wall. Oneof those is at the front and both ends of the chassis where there may asplit shear on the one side and just the positive locking on the other.

Aesthetically, the pattern cut along the seams TPS on the front arerequired. Then from above, a side view is shown as it runs along the topand down the front side edge. This illustration demonstrates therotational fastener which has the captive fastener for mounting the box.It could be a positive quarter-turn lock in which a paddle that goesbehind a phalange. A simple screw or any manner in which was mostappropriate perhaps a latching device could be used without departingfrom the spirit and scope of the invention. The enclosure system canrange from the very simplistic assembly approach which is cost effectivewhile providing high reliability and excellent from a minimization ofassembly cost with virtually no assembly or welding. This can be donewith any material in any manufacturing method—any electromagneticconductive material and any manufacturing method. In cast, molds, etc.,a three dimensional tortured path is possible, just by simply molding orcasting. In an extrusion, The TORTURED PATH™ is cut into a shape as itis extruded and used for airflow and EMI container.

) FIG. 4C is a side view of the two-dimensional EMI shielding solutionfor computer enclosures. Again, from the top, there's just the one lineacross the top front that is shown which would be exactly the same onthe bottom. There is one line on the sides and also at the front runningup and down the 1-U dimension, both at the front and back and which isall that is seen from the side. Along the top edge of the side and onthe back of the top edge, both seams are seen, near the corner on theback of both edges. Also shown in FIG. 4B is a view of the isometric ofthe rear back corner and The TORTURED PATH™ edges along the top and theside and down the back corner of the base of the chassis which are seenand then both edges of the lid come down into that corner.

is examined in detail. At the close-up of the front

there are formed tabs that are bent down and go up one plane and aroundbehind the other plane. In this case, there's one formed from the lidthat goes behind the side wall. The side wall goes behind the front faceand also the front face goes behind the lid, creating a “three-wayconvergence” or locking corner. These three pieces come together butthey nest over and under each other so they're all interlocked withoutusing any fasteners, which further reduces manufacturing cost. In eachcase, with the slot length around these tabs (or whether it goes aroundwhere the phalange which is for the mounting screw) the length is keptbelow the allowable length for a 2 GHz EMI shielding or whateverfrequency is selected to control. A hole on the top right providesadditional flexibility for this embodiment of the present invention andallows a rivet to be used to adjoin the lid to the base. In aconfiguration without a removable lid, a counter-sunk screw may be used.In this case, remove and take the lid off, take the face right off, takethe lid off and there is access to the inside of the box. As illustratedby this principle, a particularly attractive feature of the presentinvention is that no matter what feature is surrounding The TORTUREDPATH™ solution in this embodiment of the present invention, it isimplemented in such a way that there is a minimal aperture lengthmaintained and therefore controls the necessary EMI.

The illustrations respectively show the same front corner of the 17½inch (in a preferred embodiment, but dimensions are dependent onend-use) 1-RU box and it shows how the tongue tabs UTBs and LTBs wouldover-lock and interlock with one and other and would make together tobring this whole assembly together, which sort of a tongue and groovetype thing. An excellent assembly for minimizing fasteners would help toalign the chassis and could bring some electrical contacts together,although it's not dependent on it for EMI. Also, here it uses captivefasteners, both in the screwed or retaining screw, and also there is apem-nut PN, which is mounted to the back of the mounting phalange MP.This shows that once stamped and formed the features are very simple andprovided at a very low cost. This is a highly effective manner forfabricating, assembling, and maintaining EMI which is a low cost, highperformance and excellent solution.

FIGS. 4A-4D illustrate that the solution provided in the computerenclosure applications of present invention can be implemented with easein all major manufacturing methods including: stamped, laser cut, cast,extruded, molded, etc. In each manufacturing method, almost all of thebenefits of each (detailed above) will apply. Further, because there isa “gap” between mating components, the tolerances in the fabricationprocess are as “liberal” as possible. The liberal tolerances furtheraccentuate reliability and ensure the highest possible yield of partsoff the manufacturing line, so that generally, there are no fit issues.Further, the “one-hit” two-dimensional solution can improve packagingflexibility and then

as well. For example, the inventive solution may

for chassis fabrication, but also for modules, FRUs, connectors andother I/O components that require EMC protection/shielding. Theinventive solution to cut shapes to provide great open areas forairflow, does not adversely impact the EMI performance and leads to theconclusion that manufacturing cost remains low, while thermalperformance remains high.

From an improved EMI-shielding (leaking and protection) perspective, ifthe apertures are cut with maximum efficiency, the EMI without anantennae that it needs to radiate, the thermals open up(hopefully not atthe cost of EMI performance. ) One can extend this concept intoindustrial design and then start to take these cuts and make them partof the industrial design. Similar to enhancing thermals, lowering costsrelative to gaskets, screws, welds, etc. is one hundred percentreliable. There's absolutely no reliability degradation over time. Whenthese two things are brought together, there's an air gap. There are nocompression setting gaskets. There is no deforming or bending ofberyllium copper. There is no separating of foam over fabric gaskets,which separate. They are sheared, they separate, and they're bonded withadhesive or something similar. When they are sheared, they may fail. Thecompression is set over time and they lose performance over time. Theberyllium copper is outlawed in Europe. It bends and spoons bend, andthey depend on physical contact. This, however, doesn't depend onphysical contact. It's 100 percent reliable over the life of theproduct. Additionally, with the gap size set right, where the gap sizeis twice the total summation of the geometric tolerances, it will fittogether in sequence, providing a “never-fail” assembly with virtuallyno assembly defects. This is designed to have zero assembly defects. Itwill always assemble and it will be 100 percent reliable.

Twice the geometric is the normal summation of geometric tolerances. Ifthere is a gap which is double, there will be 100 percent margin ofsafety against any assembly error, any assembly defect. In thisembodiment, there is an infinite safety and an infinite reliability inassembly. There will be no assembly errors. The two-dimensionalembodiment of the invention provides a solution with no waste orfailures in assembly. A face plate will not bind or crash in front of acustomer. Inspection can be eliminated and result in lowering costs. Itis environmentally friendly with zero impact to the environment. Allpre-plate material can be used which is very important. Pure pre-platemay be used with no concern about post-plating anymore. All theentrapment issues and the environmental issues associated with the costare eliminated. With post-plates, one must take all the sheet metal. Itis necessary to ship some of it to somebody that plates it, to get itall plated and packaged up properly so it doesn't get all scratched.However, in this case, this can all be done with pre-plate material, 100percent. The only process is to ship it and do the assembly.

two-dimensional embodiment of the invention

manufacture and cost reduction in the assembly process, because thereare no welds necessary or any post-operations. When welding is part ofthe process, there must also be post-plating because it is not possibleto weld pre-plate. It would ruin the plating. Otherwise, if there is aweld and then the post-plate, the whole thing must be mapped. Theproblem is, if there is a map with a post-plate, if there are any hems,the result is entrapment; with entrapment, there exists a source foroxidation. So if the plating material is entrapped, it just sits therein the gap, or it doesn't get in at all. It either gets entrapped or itdoesn't penetrate and if there is enough safe oxidation, there iscorrosion. In this embodiment, that's all eliminated.

Referring again now to FIGS. 5A-13, a “3-D Box” or three-dimensional EMIshielding solution implementation is shown. A preferred embodiment ofthe three-dimensional implementation of the invention is discussedabove.

FIGS. 6 and 7 show different views of the five-sided box and lid,respectively of the inventive enclosure in the preferred embodiment. Nowreferring to FIG. 6, it is helpful to think of the some other planesformed by the three-dimensional shapes in both the lid and the box. Forexample, there are other “XY” planes that are formed by bottom of the“male” partial spheres in the box FSE, labeled as “plane XY(#2)”, thetop of the male partial spheres as “plane XY(#3). For purposes ofconsidering EMI shielding requirements for the electronic enclosure,other planes may be considered in view of each seam, such as the YZ andXZ planes formed at the intersection of the box FSE and the flange/lidFL. The YZ Planes formed from the seam at the respective junction of thebox FSE and the lid FL include Planes YZ(#A) and YZ(#A)′, respectively.There are also the planes formed from the inside (closest to the middleof the enclosure) part of the three-dimensional shape IP, which may bethe interior plane of the male shapes XZ#3 and the part closest to theinterior wall, as XZ#2. The XZ and YZ planes will be similar instructure and function because they are formed by the width and lengthof the enclosure. The planes are described for illustration purposes andshould not be seen as limited the scope of the invention or the possibleapplication of various embodiments.

TABLE 6 Planes formed by three-dimensional shapes in the five-sidedenclosure. Plane Description XY#A Plane formed by seam of the junctureof the lid and the five-sided enclosure, parallel to the top of thephalange. XY#2 Plane formed by the “bottom” of the male scallop andparallel to XY#A. XY#3 Plane formed by the “top” of the male scallop andparallel to XY#A XZ#A Plane formed by the width of the five-sidedenclosure perpendicular to the lid surface. XZ#2 Plane formed by theinnermost point of the male scallop and parallel to the XZ#A plane.(Generally the interior wall of the five-sided enclosure). XY#3 Planeformed by the outermost point of the male scallop and parallel to theXZ#A plane. YZ#A Plane formed by the length of the five-sided enclosureperpendicular to the lid surface. YZ#2 Plane formed by the innermostpoint of the male scallop and parallel to the YZ#A plane. (Generally theinterior wall of the five-sided enclosure). YZ#3 Plane formed by theoutermost point of the male scallop and parallel to the YZ#A plane.

Referring to FIG. 7, the planes formed by the “female” scallop orpartial spheres is shown. FIG. 7 shows a front view of the lid and thethree relevant planes and the “subplanes” formed by the female scallopor quarter-spheres for purposes of provided electromagnetic interferenceshielding. In general, these planes will be a small gap from theircounter-parts with the male three-dimensional shapes in the five-sidedenclosure. However, the XY#A plane should correspond very closely to theXY#S plane in particular embodiments. In alternate embodiments(discussed below in FIG. 10), the gaps can be adjusted to providethermal or manufacturing advantages, but may compromise some of the EMIshielding. The end-user of the alternate embodiments may have specifictradeoffs in terms of “gaps” for different electronic enclosures.

TABLE 7 Planes formed by three-dimensional shapes in the lid. PlaneDescription XY#S Plane formed by seam of the juncture of the lid and thefive-sided enclosure, parallel to the top of the phalange. XY#4 Planeformed by the “bottom” of the female scallop and parallel to XY#S. XY#5Plane formed by the “top” of the female scallop and parallel to XY#S.XZ#S Plane formed by the width of the phalange and perpendicular to thelid surface. XZ#4 Plane formed by the innermost point of the femalescallop and parallel to the XZ#S plane. XY#5 Plane formed by theoutermost point of the female scallop and parallel to the XZ#S plane.YZ#S Plane formed by the length of the phalange perpendicular to the lidsurface. YZ#4 Plane formed by the innermost point of the female scallopand parallel to the YZ#S plane. YZ#5 Plane formed by the outermost pointof the female scallop and parallel to the YZ#S plane.

Referring to FIG. 8A, detail of the “female” three-dimensionalstructures in the flange is shown. FIG. 8B illustrates the detail of the“male” three-dimensional structures in the five-sided box. As

“gap” or “fit” between the male and female

vary between particular embodiments of the invention, depending on theneeds of the end users.

Referring now to FIGS. 9A and 9B, the “torture chambers” are shown inoperation as they serve to provide electromagnetic interferenceshielding in and out of the enclosure. These interesting molded or castshapes that generally do not allow the wave to propagate and it will bereflected continually and eventually absorbed as it tries to make itthrough the interface or plane of the seam (see Plane #A, above).Because the gaps between each of these are not sufficient to formantennae on the outside of the box or through the gap, then theelectromagnetic interference is basically fully absorbed. In thisparticular illustration, as shown in FIG. 9B, the lid will have aphalange, so there will also be a wave guide effect from the phalange.In general, there will be a very short aperture which will provide waveguide effects with the “torture chambers” apertures themselves. With theshortening the aperture size and creating a tortured chamber, a waveguide and then a wave guide phalange working in concert. As illustrated,there is virtually no possibility of the EMI escaping in the chassiswith no gaskets, no fasteners intermittently around the lid. The lid isjust tacked down (or other fastening means) in a couple of places,whatever is required or, alternately, a latch in one other place. Thepreferred embodiment is very straightforward with virtually no fastenersand an excellent EMI protective container.

FIG. 10 also shows a functional diagram of the EMI protection providedby the particular embodiments of the invention. The electromagneticinterference EMI PROP is propagated in all directions from theelectronic device (not shown). The gap between the box and the lid GAPis of width g, and can vary from embodiment to embodiment. Optionally, agap adjustment structure GTA may be used to improve ventilation byincreasing the gap g. Thus, a particular embodiment may be versatileenough for different end uses. As can be seen, the plane formed by theactual seam XY#1 is “above” the plane where the formation of thestructures at the female/male interface is located XY#S+ (whichundulates between the XY#2(4) and XY#3(5)). In other embodiments (notshown), the plane formed by the seam can be across the plane formed bythe female/male interface, depending on the complexity and needs of theend user.

FIGS. 11A-11C are detailed views of the five-sided enclosure FSE in aprimary embodiment and the interior pattern IP from angled, top, andside views respectively.

As can be appreciated by those skilled in the art, otherthree-dimensional shapes may also be used, such as the single row ofcubic structures shown in FIG. 12, or the double row of cubic structuresshown in FIG. 13, without limiting the scope of the invention or thepossible embodiments. Certain three-dimensional shapes provide EMIshielding advantages, which must be weighed against the manufacturingadvantages provided by other types of shapes. Thus, while a ¼-sphere, asshown in FIGS. 5-7, will provide one type of advantage, while athree-dimension

as the cubic shape shown in FIGS. 12 and 13, will

of manufacturing simplicity;

FIG. 14 illustrates a “hybrid” embodiment of the invention incorporatingboth two and three dimensional features in combination or separately toprovide electromagnetic interference shielding for an electronicsenclosure.

1. An enclosure for a computer or other electronic device requiringelectromagnetic interference shielding (EMI) made of an electricallyconducting material, wherein said enclosure has six sides ofelectrically conducting material, wherein at least two of said sides areformed or cut such that they have a volumetric wave attenuation portioncovering at least a portion the width of the side such that saidvolumetric wave attenuation portion interferes with EMI wavepropagation, such that sufficient shielding is provided by said waveattenuation portions.
 2. The enclosure as recited in claim 1, whereinsaid volumetric wave attenuation portion is raised away from theinterior volume of said enclosure.
 3. The enclosure as recited in claim1, wherein said volumetric wave attenuation portion is lowered towardsthe interior volume of said enclosure.
 4. The enclosure as recited inclaim 1, wherein said volumetric wave attenuation portion is stamped,molded, formed, extruded, or casts.
 5. The enclosure as recited in claim1, wherein said volumetric wave attenuation portion runs along the edgeof the top surface of said enclosure.
 6. The enclosure as recited inclaim 1, wherein said volumetric wave attenuation portion includes anumber of raised features appropriately spaced to attenuate the range offrequencies that require shield.
 7. The enclosure as recited in claim 6,wherein said features include a partial sphere.
 8. The enclosure asrecited in claim 6, wherein said features include a partial cylinder. 9.The enclosure as recited in, claim 6, wherein said features include apartial box or cube.
 10. A method for providing improved electromagneticinterference (EMI) shielding for an electronic device that includes amultiple piece enclosure made of electrically conductive material, andwith at least one seam, including the acts of: creating a

-dimensional EMI shielding shapes running along at

perimeter of one of said pieces of said multiple piece enclosure; andassembling said enclosure such that said series of three-dimensional EMIshielding shapes cross the plane formed by said at least one seam. 11.The method as recited in claim 10, wherein said three-dimensional shapesare partially spherical.
 12. A multiple-piece enclosure for anelectronic device comprising: a five-sided enclosure with an interiorspace, said enclosure including an open side and configured along thetop perimeter of said open side with a first electromagneticinterference (EMI) shielding pattern, wherein said first EMI shieldingpattern includes a series of three-dimensional shapes continuous alongsaid perimeter; and a flange configured to fit onto said enclosure,creating a six-sided enclosure, said flange including a secondelectromagnetic interference (EMI) shielding pattern that includes aseries of three-dimensional shapes around the perimeter of the bottomside of said flange.
 13. The enclosure as recited in claim 12, furtherincluding connecting pieces that fit through the top of said flange intosaid perimeter of said five-sided enclosure.
 14. The enclosure asrecited in claim 12, wherein said first and second three-dimensional EMIshielding patterns are configured to be complementary such that they fittogether when said flange is placed on said five-side enclosure.
 15. Theenclosure as recited in claim 12, wherein said first and secondthree-dimensional EMI shielding patterns are cubic.
 16. The enclosure asrecited in claim 15, wherein said first pattern includes two rows ofsaid three-dimensional cubic shapes around the perimeter of saidfive-sided enclosure.
 17. The enclosure as recited in claim 12, whereinsaid first and second three-dimensional patterns are spherical.
 18. Theenclosure as recited in claim 17, wherein said first pattern includes

three-dimensional spherical shapes around the

sided enclosure.
 19. The enclosure as recited in claim 12, wherein saidfirst and second three-dimensional patterns are partially cylindrical.20. A computer enclosure providing sufficient EMI/EMC shielding whereinat least multiple sides are configured such that sinusoidal threedimensional shapes are shaped in two dimensions and then shaped againinto the orthogonal planes, such that orthogonal planes have sweepingshapes that force the EMI to traverse through narrowed apertures for theuse of shaping, wherein said material around the chassis is made ofelectromagnetic conductive material.